Failure of T cells from tumor-bearing hosts to effectively recognize and eliminate tumor cells is one of the major factors of tumor escape from immune system control. An effective antitumor immune response requires participation of the host bone marrow antigen-presenting cell (APC) responsible for the presentation of tumor-specific antigens. Dendritic cells (DC) and macrophages are the two most potent groups of APC. These cells are capable of inducing primary immune responses including the cytotoxic T-lymphocyte response.
Recent studies have clearly demonstrated that the immuno-stimulatory characteristics of DC are dependent on their maturation state. Increasing evidence supports the notion that both immune activation and immune suppression depend on antigen presentation by APC.
DC as well as macrophages and granulocytes arise from a common myeloid progenitor that has the ability to capture antigen but lacks the expression of major histocompatibility complex (MHC) class II and co-stimulatory molecules. Mature DC loaded with antigen are highly effective in eliciting a protective immune response against tumors, whereas immature DC may induce the antigen specific inhibition of CD8+ T cell effector function.
It appears that impaired balance between mature and immature myeloid cells is one of the hallmarks of cancer. There is increasing evidence that progressive tumor growth is associated with an accumulation of immature myeloid cells, monocytes/macrophages, and with a decreased number and function of DC in cancer patients as well as in tumor-bearing mice. The increased presence of immature myeloid cells (IMCs) capable of inhibiting T cells responses could be the major factor responsible for immune suppression in cancer patient.
The growth of many mouse carcinomas is associated with the early development of splenomegaly and the marked accumulation of IMC in the lymphoid organs (JAFFE et al., Mol. Med., vol. 2, p: 692-701, 1996; KUSMARTSEV et al., J. Immunol., vol. 165, p: 779-85, 2000). Decreased presence of DC in the peripheral blood of patients with breast, lung, head and neck cancer was associated with the accumulation in the peripheral blood of cells lacking markers specific for mature myeloid and lymphoid lineages (ALMAND et al., Clin. Cancer Res., vol. 6, 1755-1766, 2000). About one third of these cells were immature macrophages and DC and the remaining cells were IMC at earlier stages of differentiation (ALMAND et al., J. Immunol., vol. 166, 678-689, 2001). The peripheral blood presence of these cells was dramatically increased in patients with advanced stage cancer, but dropped considerably within three to four weeks after surgical resection of the tumor. This finding is consistent with the hypothesis that the generation of IMC is due to the production of soluble factors by tumors.
Consistent with this hypothesis, it has been shown that several tumor-derived factors affect DC maturation from hematopoietic progenitor cells (HPC).
VEGF is produced by most tumors, and its production is closely associated with poor prognosis (TOI et al., Eur. J. Cancer, vol. 32A, p: 2513-9, 1996). It has been shown that neutralizing anti-VEGF antibody blocked the negative effects of tumour cell supernatants on DC maturation in vitro (GABRILOVICH et al., Nat. Med., vol. 2, p: 1096-103, 1996). Moreover, a continuous in vivo VEGF infusion results in a dramatic inhibition of DC production (GABRILOVICH et al., Blood, vol. 92, p: 4150-4166, 1998).
VEGF inhibits the activation of transcription factor NF-κB in hematopoietic progenitor cells, which is accompanied by alterations in the development of multiple lineages of hematopoietic cells (DIKOV et al., Cancer Res., vol. 61, p: 2015-21, 2001). Chronic administration of recombinant VEGF in naïve mice results in an inhibition of DC development and in an increased production of B cells and immature GR-1+ myeloid cells (OYAMA et al., J. Immunol., vol. 160, p: 1224-32, 1998).
GM-CSF is another factor that has been shown to be responsible for the stimulation of myelopoiesis in tumor-bearing host. The chronic administration of GM-CSF to mice results in the generation of a cell population that morphologically resembles granulocyte-monocyte progenitors that express the granulocyte-monocyte markers Mac-1 and Gr-1 (BRONTE et al., Blood, vol. 96, p: 3838-46, 2000).
Other tumor-derived factors such as M-CSF, IL-6 (MENETRIER-CAUX et al., Blood, vol. 92, p: 4778-4791, 1998), and IL-10 (ALLAVENA et al., Eur. J. Immunol., vol. 28, p: 359-69, 1998; FAULKNER et al., Immunology, vol. 99, p: 523-31, 2000) or gangliosides (SHURIN et al., Cancer Res., vol. 61, p: 363-9, 2001) have also been involved in defective DC differentiation in vitro. Neutralizing anti-IL-6 and anti-M-CSF antibodies abrogate the negative effect of supernatants from renal cell carcinomas on DC differentiation (MENETRIER-CAUX et al., above-mentioned, 1998). However, it appears that these factors do not stimulate myelopoiesis and mostly affect relatively mature cells. Moreover, IL-10 appears to prevent the differentiation of monocytes to DC, but promotes their maturation to macrophages (ALLA-VENA et al., above-mentioned, 1998). Furthermore, ALMAND et al. (2000, above-mentioned) has shown that only patients with peripheral blood elevated levels of VEGF showed statistically significant increased of IMCs after measuring the levels of M-CSF, GM-CSF, IL-6, IL-10, TGF-β, and VEGF.
The characterization of IMCs has shown that these cells actively suppress Ag-specific T cells responses and contributes to tumor nonresponsiveness (ALMAND et al., above-mentioned, 2001). Thus, IMCs actively inhibit CD8+ T cell effector function, MHC class-II associated T-specific proliferation and MHC class I-restricted IFN-γ production in the presence of functionally competent DC. It has been suggested that physiologically, IMCs may serve as a defense mechanism that limits the expansion of activated T cells and prevents the development of autoimmune diseases. However, in the case of cancer, the accumulation of IMCs may lead to the profound suppression of immune responses.
Consequently, IMCs depletion or differentiation constitutes actually an important strategy to improve immune response in cancer.
Different ways of therapies have been explored by in vitro IMCs differentiation experiments. Thus, it has been shown that a combination of ATRA and GM-CSF is able to induce the differentiation of the majority of IMCs into relatively mature DC (ALMAND et al., above-mentioned, 2001). ATRA is a naturally occurring isomer of retinoic acid that is successfully used in differentiation induction therapy in patients with acute promyelotic leukaemia (CASTAIGNE et al., Blood, vol. 76, p: 1704-9, 1990).
However, if the general in vivo induction of the differentiation of IMCs by an adapted factor could induce a tumor remission, it could also induce the development of an autoimmune response.
It could therefore be advantageous to provide a composition for inducing IMCs differentiation within the tumor sites to induce tumor remission without any side effects.